Fluorescence Quenching Effects of Copolymers by Surface-Modified

In the fluorescence quenching studies, where acid-donor-modified AuSCOOH nanoparticles and non-acid-modified AuSC10 nanoparticles (diameter ca. 5 ~ 6 nm) were doped, the photoluminescence (PL) spectra of copolymers P1 and P4 were monitored in the presence of different concentrations of surface-modified gold nanoparticles (Figure 3.5). The fluorescence spectra of pure copolymers P1 and P4 exhibit maximum PL emissions at 447 nm and 431 nm with corresponding quantum yields of 43% and 58%, respectively. In Figure 3.5(a) and 3.5(b), copolymers P1 and

P4 demonstrated dramatic decreases in fluorescence intensities upon the addition of

AuSCOOH nanoparticles (which contain 21% carboxylic acid surfactants). Upon the

addition of AuSCOOH, progressive fluorescence quenching effects on copolymer P1 (blended with AuSCOOH) were observed by increasing the concentration of

AuSCOOH. It is conceivable that the polymer nanocomposite P1-AuSCOOH reduced the fluorescence emission intensities by H-bonding complexation of the fluorescent pyridyl units with the acid surfactant on AuSCOOH. The H-donor (para-benzoic acid) units in self-H-bonded copolymer P1 were easily H-bonded with the pyridyl H-acceptor groups and competed with the acid H-donor surfactants on

AuSCOOH nanoparticles. However, as shown in Figure 3.5(b), the titration of

acid-protected copolymer P4 by AuSCOOH nanoparticles displays a similar PL quenching trend as Figure 3.5(a). This suggests that the complexation of acid-protected copolymer P4 and AuSCOOH nanoparticles has a similar H-bonding interaction as that in the nanocomposite P1-AuSCOOH. Unlike the self-H-bonded copolymer P1, there is no H-donor competition from the acid-protected copolymer P4 during the H-bonding complexation process in the nanocomposite P4-AuSCOOH.

Hence, the excitons of acid-protected copolymer P4 are more easily trapped by the charge-transfer quenchers of AuSCOOH than those of the self-H-bonded copolymer

P1. As a comparison, the solutions of copolymers P1 and P4 were titrated with

another nanoparticle counterpart, alkyl-functionalized gold nanoparticles (AuSC10, which bears acid-free surfactants). Both PL titrations of copolymers P1 and P4 by

AuSC10 nanoparticles (Figure 3.5(c) and 3.5(d)) resulted in similar PL reductions

after increasing the concentration of AuSC10 nanoparticles. However, due to less H-bonding interactions of non-acid-modified AuSC10 nanoparticles (containing acid-free surfactants) with copolymers P1 and P4, AuSC10 nanoparticles have much weaker PL quenching effects on P1 and P4 than acid-modified AuSCOOH nanoparticles (Figure 3.5(a) and 3.5(b), respectively). Comparing the insets of Figure 3.5(a)-(d), the fluorescence quenching curves for copolymers P1 and P4 titrated by

AuSCOOH (Figure 3.5(a)-(b)) are totally different from those titrated by AuSC10

(Figure 3.5(c)-(d)). The lower quenching effects observed for PL titrations of

AuSC10 nanoparticles on copolymers P1 and P4 are owed to the absence of

H-bonding interactions between copolymers (P1 and P4) and the acid-free surfactants on AuSC10 nanoparticles.

The PL quenching behavior follows the Stern-Volmer relation I0/I = 1+KSV[Q],⁹⁶ where I0 and I are the emission intensities of the fluorescent copolymers (P1 and P4) in the absence and presence of the quencher Q (surface-modified gold nanoparticles), respectively, KSV is the Stern-Volmer quenching constant, and [Q] is the concentration of the quencher. Figure 3.6 demonstrates Stern-Volmer plots of copolymers P1 and P4 for various concentrations of acid-modified AuSCOOH nanoparticles and non-acid-modified AuSC10 nanoparticles, which are replotted from the insets of Figure 3.5(a)-(d). The quenching constants (Ksv) of copolymers P1 and

P4 titrated with different nanoparticle quenchers (AuSCOOH and AuSC10) in THF solutions are obtained from the slope of Figure 3.6 and listed in Table 3.4. In comparison with the fluorescence quenching effects of AuSCOOH nanoparticles on copolymers P1 and P4, the quenching constant (Ksv = 1.20 x 10⁵ M^-1) of P1 is smaller than that (Ksv = 1.41 x 10⁵ M^-1) of P4. This can be explained by that the partial self-quenching effect of H-bonds in copolymer P1 (with a lower quantum yield than non-self-H-boded copolymer P4) is induced by the higher aggregation of

luminescent H-acceptor moieties in P1 H-bonded to its own H-donor moieties.

Similar to the previous results of copolymers P1 and P4 titrated with AuSCOOH, as copolymers titrated with AuSC10 nanoparticles, the quenching constant (Ksv = 1.32 x 10⁴ M^-1) of P1 is slightly smaller than that (Ksv = 1.57 x 10⁴ M^-1) of P4. Due to reduced supramolecular interactions between AuSC10 and copolymers P1 and P4 in the nanocomposites, both Ksv constants of copolymers P1 and P4 (1.32 and 1.57 x 10⁴ M^-1 without H-bonds) blended with non-acid-modified gold nanoparticles (AuSC10) are much smaller than those (1.20 and 1.41 x 10⁵ M^-1 with H-bonds) H-bonded with acid-donor-modified gold nanoparticles (AuSCOOH). Hence, the H-bonding interactions play an important role in our study of the fluorescence quenching effect. Moreover, it will be more interesting to develop a multicomponent self-assembly process involving fluorescence quenching of pyridyl H-acceptors specifically by both acid H-donors from copolymer P1 and acid-donor-modified gold nanoparticles (AuSCOOH). In general, the significant supramolecular interactions between various surface-modified gold nanoparticles and fluorescent polymers can be distinguished by the distinct fluorescence quenching behavior with specific quenching constants (Ksv). In order to confirm the fluorescence quenching effects on copolymers

P1 and P4 by AuSCOOH, UV-visible absorption analyses of copolymer

nanocomposites, containing AuSCOOH, were carried out. We would expect the

UV-visible absorption spectra to change if the aggregation of fluorescent polymers is induced by the addition of AuSCOOH. In the UV-visible absorption spectra, the absorption peaks of copolymers P1 and P4 (350 and 520 nm in THF solutions) titrated by AuSCOOH do not red shift with increasing Au nanoparticle concentrations (from 0 to 121 μM). These were the same processing conditions used for the quenching titrations of fluorescent polymers P1 and P4 by Au nanoparticles.

The reasonably low concentrations (smaller than 121 μM) of AuSCOOH were maintained to avoid the aggregation of fluorescent copolymers and AuSCOOH nanoparticles. Therefore, the aggregation effects on fluorescence quenching can be ignored. The major source of fluorescence quenching is then attributed to energy transfer between fluorescent copolymers (P1 and P4) and Au nanoparticles (AuSCOOH).

Table 3.4 Stern-Volmer Constants (Ksv) of Copolymers P1 and P4 Titrated with Different Nanoparticle Quenchers (AuSCOOH and AuSC10) in THF Solutions

Ksv (M^-1)^a

P1 P4

AuSCOOH 1.20 x 10⁵ 1.41 x 10⁵

AuSC10 1.32 x 10⁴ 1.57 x 10⁴

a The quenching behavior follows the Stern-Volmer relation I0/I = 1+KSV[Q],where I0

and I are the emission intensities of the fluorescent copolymer (P1 or P4) in the absence and presence of the quencher Q (surface-functionalized gold nanoparticles), correspondingly, KSV is the Stern-Volmer quenching constant, and [Q] is the concentration of the quencher.

(a) (b)

(c) (d)

Figure 3.5 Fluorescence quenching spectra of copolymers P1 and P4 titrated by surface-functionalized nanoparticles (AuSCOOH and AuSC10) in THF solutions: (a) P1 and (b) P4 by varying the concentration of acid-donor-modified gold nanoparticles (AuSCOOH); (c) P1 and (d) P4 by varying the concentration of non-acid-modified gold nanoparticles (AuSC10).

Figure 3.6 Corresponding Stern-Volmer plots of copolymers P1 and P4 for increasing concentrations of acid-modified gold nanoparticles (AuSCOOH) and non-acid-modified gold nanoparticles (AuSC10) in THF solutions.

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